How to bake roughness into green channel only for usage with GLTF (Blender 2.8)
ok I do kinda understand what you want to achive, you need to do it differently though to get the best possible outcome.
you need a CombineRGB node, plug the Rougthness in R, the Metallic in G and the AO in B (if you want to use the Red channel of the diffuse as the rougthness) and then you plug that in to a diffuse node, this then to the output


then you go over to the baking section, there you select Diffuse (COLOR)


Create your image in the image editor and put it into the material you want to bake, make sure its selected


if you now bake it, you will see that it works, it only uses the color of the diffuse node, not shades, nothing else, just color.

to get the data back you will simply use a SeperateRGB node.